class Solution {
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {1, -1, 0, 0};
    bool vis[9][9];
    int m, n, sz;
public:
    bool exist(vector<vector<char>>& board, string word) {
        m = board.size(), n = board[0].size(), sz = word.size();
        for(int i = 0; i < m; ++i)
        {
            for(int j = 0; j < n; ++j)
            {
                if(board[i][j] == word[0])
                {
                    vis[i][j] = true;
                    if(dfs(board, i, j, word, 1) == true)
                        return true;
                    vis[i][j] = false;
                }
            }
        }
        return false;
    }

    bool dfs(vector<vector<char>>& board, int i, int j, string& word, int pos)
    {
        if(pos == sz)
            return true;
        for(int k = 0; k < 4; ++k)
        {
            int x = i + dx[k], y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] && board[x][y] == word[pos])
            {
                vis[x][y] = true;
                if(dfs(board, x, y, word, pos + 1) == true)
                    return true;
                vis[x][y] = false;
            }
        }
        return false;
    }
};